In the design and manufacture of semiconductor IC (integrated circuit) chip packages and modules, it is imperative to consider and implement mechanisms that can effectively remove heat that is generated during operation of the IC chip devices (e.g., transistors). Indeed, the operating temperatures of chip devices must be maintained low enough to ensure continued reliable operation of such devices. The ability to efficiently remove heat becomes even more problematic as chip geometries are scaled down and operating speeds are increased, resulting in increased power density. As such, the ability to effectively cool semiconductor chips is a factor that limits increases in system performance.
There are various techniques that have been developed for removing heat from semiconductor packages. For example, one technique that is typically employed includes thermally coupling a heat sink or cooling plate to one or more semiconductor IC chips using a compliant thermally conductive material. The cooling plate or heat sink, which is typically formed of a high thermal conductivity material, such as copper or aluminum, will conduct heat away from the IC chip(s) and the heat is removed from the cooling plate or heat sink by methods such as forced air cooling or circulating liquid coolants.
A compliant thermally conductive material is used to thermally couple the IC chip and cooling plate (as opposed to a rigid bond) when, for example, the difference in thermal expansion between the material of the IC chip and the material of the cooling plate or heat sink is relatively large. A cooling plate, or high performance heat sink, or package lid, is typically made of copper (Cu), which has thermal coefficient of expansion (TCE) of about 16.5 ppm/° C., which is substantially larger than the TCE of Silicon (Si) which is about 2.5 ppm/° C. The layer of compliant thermally conductive material reduces stress at the thermal connection due to differences in thermal expansion of the IC chip and heat sink.
Compliant thermally conductive materials include, for example, thermal pastes, thermal greases, or thermally conductive fluids such as oils, and are frequently referred to as thermal interface materials, or TIMs. Thermally conductive pastes typically comprise thermally conductive particles having a distribution of sizes dispersed within a binder material or matrix (such as the paste described in U.S. Pat. No. 5,098,609). A thermally conductive paste can be applied between the top of an IC chip that is mounted on a substrate and a lower flat surface of a cooling plate facing the substrate. Typical TIMs include those having a wax matrix, commonly known as phase-change materials, those having a silicone-based matrix, and dry particle lubricants such as graphite and metal powders. Less viscous thermally conductive materials, such as oils, have a lower thermal conductivity than pastes, but can also be applied in much thinner layers, resulting in improved thermal performance, but less mechanical compliance.
Other conventional techniques for attaching a heat sink to a semiconductor chip package include bonding a heat spreader directly to a non-active surface of an IC chip using a thermally conductive rigid bonding material and then thermally coupling a heat sink to the heat spreader using a compliant thermally conductive material.
Indeed, when the thermal expansion match of the materials that form the heat spreader and IC chip are closely matched, a rigid bond may be used to thermally couple the heat spreader to the IC chip. More specifically, by way of example, in a single chip module (SCM) type package comprising a silicon IC chip, a thermal spreader composed of a high thermal conductivity material with a thermal expansion coefficient close to that of Si, such as SiC (TCE of ˜4 ppm/° C.) or diamond (TCE of ˜2.8 ppm/° C.), can be rigidly bonded to the IC chip using a silver filled epoxy, filled polymer adhesive, filled thermoplastic or solder, or other thermally conductive bonding material. Polymer materials for thermally conductive bonds may be filled with particles of any material with a high thermal conductivity. A rigid bond typically has a lower thermal resistance than a layer of compliant thermally conductive material. The ability to effectively use a rigid bond is limited not only by the difference in the TCEs of the materials that form the heat spreader and IC chip, but also on the temperature range (cycle) in which the semiconductor package will operate or be exposed to, as well as size of the area over which the rigid bond will be formed.
A heat sink can then be mounted onto the thermal spreader using a layer of a compliant thermally conductive material. In some packages, the package cap, or lid, acts as a thermal spreader and a heat sink is subsequently attached to the top surface of the package cap or lid.
Moreover, effective heat removal is difficult for densely packed, high-powered devices. For example, a multi-chip module (MCM) package which comprises an array of IC chips mounted face down on a common substrate, presents special cooling difficulties. In an MCM package, the IC chips may be mounted very close together and nearly cover the entire top surface of the MCM. With such an arrangement, it may not be possible to use a heat spreader bonded directly to the back surface of the chips, as is sometimes used for isolated chips, to reduce the heat flux (power/unit area, i.e. W/cm2).
Furthermore, difficulties arise for efficient heat removal with respect to IC chips, such as processors, that have “hot spot” regions, which can have a heat flux significantly greater than the average heat flux, resulting in temperatures ˜20° C. hotter than the average chip temperature. A thermal solution that may be adequate for efficiently removing heat that is generated due to average chip power density may not be adequate for removing heat at “hot spot” regions of the chip, which can result in failure of the chip devices in or near the “hot spot” region.
FIG. 1 is a diagram that schematically illustrates a conventional apparatus for thermally coupling a semiconductor chip to a heat sink using techniques such as disclosed in U.S. Pat. No. 5,838,065. The apparatus (10) includes a IC chip (11) having a plurality of fins (12) formed on a non-active surface of the chip (11). In addition, the apparatus comprises another substrate (13) having a plurality of fins (14) formed on one surface of the substrate (13) and a heat sink (15) is thermally coupled (16) to another surface of the substrate (13). The material of the substrate (13) is selected to have a TCE that closely matches the TCE of the material of the IC chip (11). A thermal connection is formed by interleaving the fins (12) and (14) of the substrates (11) and (13) and bonding the two substrates together, with a silicon dioxide or fusion bond, for example.
In general, the conventional methods described above do not provide means for forming a low thermal resistance interface directly between an IC chip and a heat sink formed of a highly thermally conductive material such as copper, where the heat is dissipated to the ambient. For example, although a thermal connection can be formed by applying a compliant thermally conductive material between a back surface of a chip and surface of cooling plate or heat sink or lid made of copper or aluminum, for example, there are factors that limit the thermal conductivity that can be obtained with such thermal connection.
For instance, the thermal resistance of the paste or fluid layer can be reduced by increasing the thermal conductivity of the material, reducing the thickness of the layer, or increasing the surface area of the thermal joint. However, the thermal conductivity of a paste is limited by the volume fraction which can be occupied by solid particles while still providing adequate mechanical compliance. For an MCM, a typical thermal paste thickness (formed between 2 flat mating surfaces) is about 4 mil as it is necessary to provide adequate mechanical compliance in the structure so that micro solder balls (e.g., C4's) which provide electrical connections from the chip to the substrate are not crushed, and it is necessary to keep the thermal paste from being squeezed out of the thermal joint during cyclic loading (i.e. paste pumping), leading to an increased thermal resistance. Moreover, the thickness of thermally conductive fluids is limited by the roughness of the surfaces being joined and the sizes of the particles, if any, in the fluid, with a typical thickness being about 0.5 mil.
Moreover, although the use of rigidly bonded interleaved fins (12, 14) in the conventional package structure of FIG. 1 can provide a low thermal resistance path from the chip (11) to a TCE matched substrate (13), the thermal joint (16) between the substrate (13) and the heat sink (15) is not improved. Thus, the package (10) in FIG. 1 does not provide means for forming a low thermal resistance interface directly between an IC chip and a heat sink formed of a highly thermally conductive material such as copper.
Moreover, the conventional methods described above do not provide thermal connections that can locally reduce the thermal resistance over chip “hot-spots”, to provide increase thermal cooling for chips with non-uniform power densities.